Measuring and monitoring a power source

ABSTRACT

The disclosed battery monitoring systems and methods may minimize measurement errors due to noise and/or other disruptions. The sensor may generate a pulse width modulated signal that when applied to the battery forms an AC test signal having a defined waveform, frequency, amplitude, and/or duration. The sensor may measure the battery&#39;s response to the test signal. The resultant response signal may be measured to determine the health of the battery, including its internal admittance. The sensor may determine an optimum frequency to test the battery by scanning a frequency range and measuring the amount of noise present.

BACKGROUND

Many industries and applications use storage batteries. Power utilities,transportation companies, alternative energy companies, data centers,banks, office complexes, etc. use storage batteries to backup a primarypower source. For example, communications and computing equipment intelecommunication central offices, datacenters, and wireless networkcell sites use a direct current (DC) power source, typically −48V or−24V. Typically, the commercial alternating current (AC) power feed thatsupplies a site is converted to DC to appropriately match therequirements of the individual equipment.

Because commercial AC power is not always reliable enough for missioncritical systems, it is common to provide backup power in the event of afailure of the commercial feed. The backup power is typically providedby a collection of power sources or electrochemical batteries, known asa battery plant. The batteries may be located within the site andcoupled with power distribution bars, to feed DC equipment, and withinverters, to feed AC equipment. The batteries may be located within anuninterruptible power supply (UPS). Regardless of where the batteriesare located, they are often oriented in groups, wired in series up to arequired voltage.

In order to obtain the maximum battery run-time and life expectancy, itis necessary to perform periodic maintenance tests. These tests arecommonly performed by maintenance personnel who travel to the remotesite. Using complex and expensive equipment, the personnel takemeasurements and readings from the batteries. This work is made moredifficult when the batteries are located in areas with electromagneticnoise, such as that emitted by the equipment being powered. In order toincrease battery plant reliability and lower maintenance costs, it isdesirable to perform these tests in a manner that minimizes measurementerrors and perhaps without the need to physically dispatch maintenancepersonnel.

SUMMARY

The disclosed battery monitoring systems and methods may be used toremotely measure the condition of one or more batteries. The systems andmethods may minimize measurement errors due to noise and/or otherdisruptions, such as noise caused by related equipment.

One or more battery monitoring sensors may be mechanically andelectrically attached to the terminal posts of the monitored batteries.Each respective battery sensor may measure attributes of a respectivebattery. The attributes may include internal admittance, voltage,current, temperature, etc. The attributes may be used to determineinternal battery characteristics. The internal battery characteristicsmay be correlated with battery performance and/or overall batterycondition.

The sensors may each generate a pulse width modulated signal that whenapplied to the battery from an AC test signal having a defined waveform,frequency, amplitude, and/or duration. The sensor may measure thebattery's response to the test signal. The resultant response signal maybe measured to determine the health of the battery, including itsinternal admittance.

The sensor may determine an optimum frequency to test the battery. Thesensor may scan a frequency range and measure the amount of noisepresent. The sensor may select a frequency with little noise for testingthe battery.

By testing the battery, it may be determined that the battery isovercharged. The state of charge of individual batteries within a stringof similar batteries may be equalized by varying the duration andregularity of the battery test process, drawing current to effectivelyprotecting the batteries from overcharging.

A site control unit may be electrically attached to the battery sensorsby way of interconnection cables. The site controller may poll one ormore of the battery sensors periodically. The poll may collectmeasurement data from the sensors and/or send control commands to thesensors. The site controller may include an network interface. Thenetwork interface may enable connection to a general purpose datanetwork. The site controller may interface with software/server systems,via the data network. The software/server systems may use the sensorinformation as part of an overall battery population monitoring andmanagement program.

The disclosed measurement systems and methods provide a range ofmeasurement types, able to adapt the measurement device to a wide rangeof battery types. The disclosed system provides improved accuracy in asimplified and reproducible digital implementation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example battery monitoring system.

FIG. 2 depicts an example circuit diagram of battery model test circuit.

FIG. 3 depicts an example circuit diagram of an example battery sensor.

FIGS. 4A & 4B depict an example sinusoidal signal and a correspondingpulse width modulated signal, respectively.

FIG. 5 depicts a signal trace of an example pulse width modulated testsignal.

FIG. 6 depicts an example excitation signal, as measured from a batteryunder test.

FIG. 7 depicts an example filtered excitation signal.

FIG. 8 depicts an example timing diagram for sampling.

FIG. 9 depicts an example flow chart for testing an internalcharacteristic of a battery.

DETAILED DESCRIPTION

FIG. 1 depicts a battery monitoring system. One or more batterymonitoring sensors 105A-B may be mechanically and electrically attachedto the terminal posts of the monitored batteries 107A-B. Each respectivebattery sensor 105A-B may measure attributes of a respective battery107A-B. The attributes may include the internal admittance (i.e., theinverse of impedance), the voltage, the current, the temperature, etc.

A site control unit 102 may be electrically attached to the batterysensors 105A-B by way of interconnection cables 102A-B. The site controlunit 102 may be connected to the battery sensors 105A-B via a wirelesschannel including radio frequency (RF), optical, magnetic fieldinduction, or the like. The site control unit 102 may poll one or moreof the battery sensors 105A-B periodically. The poll may collectmeasurement data from the sensors 105A-B and/or send control commands tothe sensors 105A-B. The site control unit 102 may include a networkinterface. The network interface may enable connection to a generalpurpose data network 101. The data network 101 may be an InternetProtocol (IP) network, or it may use another network protocol. The datanetwork may employ T1, ISDN, DSL, broadband, Ethernet, WiFi, or othertransport suitable for data transfer. The site control unit 102 mayinterface with software systems, via the data network 101. The softwaresystems may use the sensor information as part of an overall batterypopulation monitoring and management program. With this system, anynumber of batteries 107A-B may be monitored.

As shown, two strings of batteries 107A-B may be monitored by anexemplary remote battery monitoring system. Batteries 107A may beconnected in series to form a first battery string. Likewise, batteries107B may connected in series to form a second battery string. Eachbattery 107A-B in each string may be connected to a sensor unit 105A-B.Each sensor unit within the first string 107A may be interconnected viaa communications cable 104A. Likewise, each sensor unit within thesecond string 107B may be interconnected via a communications cable104B. Communications cables 104A-B may be four-conductor telephonecables or any other cable suitable for data transmission. Communicationcables 104A-B enable data communication between the sensor units 105A-Band a site control unit 102. This communication may be serial and/orparallel data communication. Strings of sensor units 105A-B may beconnected to the site control unit 102. The site control unit 102 mayinclude a number of interfaces 103 to support many communication cables104A-B and, in turn, many batteries 107A-B under test. The site controlunit 102 may be connected to a data network 101.

Each sensor unit 105A-B may test its respective battery 107A-B and maycommunicate data indicative of the battery's condition (e.g.,admittance, temperature, voltage, etc.) to the site control unit 102.The site control unit 102 subsequently performs mathematicalcalculations on the received data to report metrics indicative ofbattery condition. The site control unit may report the battery metricsvia the data network 101. It may provide a regular report via FileTransfer Protocol (FTP), Hypertext Transport Protocol (HTTP), and/oranother protocol. It may provide the metrics as requested or polled by auser or management system via Simple Network Management Protocol (SNMP)and/or another protocol. The site control unit 102 may include a webserver to display battery metrics and to receive management controls.

Each sensor unit 105A-B within a string may determine the total voltageof the respective string of batteries 107A-B. The site controller 102may measure this voltage with respect to the string of batteries'overall minus voltage. The result is an overall voltage of the string.The site controller 102 may report this voltage to all sensors 105A-Bwithin the string via a broadcast message over the communication cable104A-B.

Each sensor 105A-B may determine its relative position within therespective string of batteries 107A-B. Once each sensor unit 105A-Bdetermines its relative position within the string, it may assume alogical address for purposes of communications with the site controller102. Thus, the system may identify an individual battery from thepopulation of batteries under test.

FIG. 2 depicts a circuit diagram of a battery model test circuit. Asimplified model of an electrochemical battery may include an idealvoltage source (Vb 202), two significant resistance elements (Ri 204 andRm 206), and a capacitive element (C 208) arranged as shown in FIG. 2.Other arrangements of Ri 204, Rm 206, and C 208 may be used. The modelas shown is commonly called the “Randles Battery Model.”

In this model, the ideal voltage source may be represented as “Vb” 202.An equivalent internal resistance attributable to the Vb source 202 maybe represented as “Ri” 204. The resistance due to metallic connectionsmay be represented as “Rm” 206. The equivalent internal capacitance maybe represented as “C” 208.

For example, in practical applications of lead-acid batteries withcapacities of 100-200 ampere-hours, Ri 20 and Rm 206 combined may bevalued at between about 1.0 milliohm to about 10 milliohm. Rm 206 may beapproximately 45% of the total internal resistance. Ri 204 may beattributed to current generating limitations of the electrochemicalprocess. Ri 204 may be approximately 55% of total internal resistance. C208 may be valued at about 1.5 Farads per 100 ampere-hours of capacity.Rm 206 may be attributed to the internal metallic connections.

Applying an AC current from an AC current source 210 of known frequencyand amplitude and observing the resultant AC voltage developed at thebattery terminals may enable a determination of values of the parametersof the circuit model. A sinusoidal AC test current (Itest) of knownfrequency and amplitude may be applied to the battery terminals. The ACcomponent of the voltage developed across the battery terminals (Vt) maybe a frequency dependent function. The AC component of the voltagedeveloped across the battery terminals (Vt) may relate to the values ofRi 204, Rm 206, C 208, the amplitude of the AC test current (Itest), andthe frequency of the AC test current (Itest).

For example, at low frequencies, approaching DC, the AC component of Vtmay reach a maximum value determined by the combined resistance of Ri204 and Rm 206 (Rb=Ri+Rm). At high frequencies, approaching infinity,the low reactance of capacitor C 208 may form an effective short-circuitsuch that Vt is primarily determined by Rm 206. Thus, the values of Ri204, Rm 206, and C 208 may be characterized by varying the frequency ofthe test signal while observing the magnitude of the AC component of Vt.

To illustrate, in a typical lead-acid battery, this process may besimplified by use of the following approximations:

C=1.5F per 100 amp-hours of battery capacity

1/(Rb)=150 mho per 100 amp-hours of battery capacity

Ri=1.25*Rm

These approximations generally scale linearly with amp-hour capacity.These constants may be stored in memory and used to calculate theinternal battery characteristics. This linear scaling may provide anacceptable first-order approximation of a lead-acid battery'sperformance when subjected to the AC test current as described above.The Randles circuit equivalent, shown in FIG. 2, may be modeled with asoftware circuit analysis tool, SPICE for example. In the modeling, thefrequency response of Vt may be shown to be relatively flat atfrequencies below about 25 Hz and to reduce by about 50% as the testfrequency is increased to 60 Hz. Thus, an example test frequency mayinclude up to the 20 Hz range. Since Ri 204, Rm 206, and C 208 varyfairly linearly with a battery's amp-hour capacity, this generalizedapproach is useful for a wide range of battery sizes.

FIG. 3 depicts a circuit diagram of an example battery sensor 300. Thesensor 300 is merely one embodiment; other circuits that measure similarparameters are contemplated. The sensor 300 may generate a low-frequencysinusoidal test signal by using high frequency pulse-width modulatedrectangular signals to excite the battery under test. This method mayachieve a low frequency sinusoidal test signal excitation usinghigh-frequency rectangular signals.

The sensor 300 may be connected to a battery 301 under test. Thepositive terminal 303 of the battery 301 may be connected to a firstnode of a resistive load 304 with a resistance value of Rt. The secondnode of the resistive load 304 may be connected to one contact of aswitch 314. The second contact of the switch 314 may be connected to thenegative terminal 302 of the battery 301. The switch 314 may be a singlepole single throw switch and may open and close current flow from thebattery 301 across the resistive load 304. The switch 314 may becontrollable, such as a FET switch implementation, for example.

One node of a capacitor 313 may be connected to the positive terminal ofthe battery 301. The other node of the capacitor 313 may be connected tothe input of a voltage amplifier 312. The voltage amplifier 312, mayamplify changes in voltage Vt measured at the positive terminal 303 ofthe battery 301 with respect to the negative terminal 302 of the battery301.

The output of the voltage amplifier 312 may be input to amicrocontroller 310. The microcontroller 310 may be a single-chipmicrocontroller, for example. The microcontroller 310 may include abuilt-in analog-to-digital converter. The microcontroller 310 mayinclude a dual-port optically isolated serial communications interfacewith a first port 308 and a second port 309.

In an embodiment, the sensor 300 may include a diode 307. The anode sideof diode 307 may be connected both to the microcontroller 310 and thepositive terminal 304 of the battery 301. The cathode side of the diode307 may be connected both to the microcontroller 310 and the first 308and second 309 communications interface ports. The diode 307 in thisarrangement may provide a summing function, such that when the sensor400 is placed in a string with other like sensors, the sensor 300 maydetermine its position within the string.

In an embodiment, the sensor 300 may include a resistor 306 inconnection with a diode 311 to measure temperature. A first node of theresistor 306 may be connected to a voltage regulator 305. A second nodeof the resistor 306 may be connected to both the microcontroller 310 andthe anode side of the diode 311. The cathode side of the diode 311 maybe connected to the negative terminal of battery 301. In thisarrangement, the DC voltage across the diode 311 at the microcontrollermay be proportional to the battery's temperature. Other temperaturesensors, such as an electrical resistance thermometer, an infraredthermometer, a silicon bandgap temperature sensor, a thermistor, athermocouple, a coulomb blockade thermometer, etc. may be used as well.

The voltage regulator 305 may provide DC power to the remote measurementsensor 300. One node of the voltage regulator 305 may be connected tothe positive terminal 303 of the battery 301. The other node of thevoltage regulator 305 may be connected to the temperature sensor 306,the voltage amplifier 312 and the microcontroller 310. The voltageamplifier 312 and the microcontroller 310 may also be connected to thenegative terminal 302 of the battery 301.

The output voltage of the amplifier 312, the voltage of the temperaturesensor 306, and the voltages at both sides of the summing diode 307 maybe input to the analog-digital converter in the microcontroller 310.Each voltage may be measured, quantified, and used in the mathematicaland logical processes of characterizing the battery 301.

The microcontroller 310 may be programmed to activate the switch 314.For example, the microcontroller 310 may cause a current (Itest) to flowin the battery when the switch FET gate is driven high. The currentItest may be determined by the battery voltage (Vb) and the value of Rtaccording to the formula Itest=Vb/Rt.

FIGS. 4A & 4B depict an example sinusoidal signal 402 and acorresponding pulse width modulated signal 404, respectively. Themicrocontroller may have a stepwise approximation of a sinusoid signal402 stored as a lookup table in its nonvolatile memory. For example, thestepwise approximation may include 256 points. More or fewer points inthe lookup table can be used with varying resolution.

The microcontroller may generate a pulse width modulated signal 404 thatcorresponds to the sinusoidal signal. The microcontroller may includesoftware or hardware to implement a pulse width modulator (PWM). The PWMmay step through the values in the sinusoid lookup table. Themicrocontroller may activate the FET switch for a time proportional toeach table value.

The switch may be activated (or closed) for a time (Ton) and thendeactivated (or opened) for a time (Toff) such that the duty cycle ofthe test signal varies in accordance with the stepwise approximation.The duration of Ton and/or Toff may be determined by the sinusoid valuesfrom the look up table. Each on-off cycle (i.e., Ton+Toff) may have aconstant time period (Tperiod). The value Tperiod may be determinedaccording to the desired low frequency test sinusoid frequency (Ft) andthe number of points (Np) in the sinusoid lookup table. For example,Tperiod may be equal to the result of 1/(Ft*Np).

FIG. 5 depicts a signal trace of an example pulse width modulated testsignal 502. The microcontroller may generate the pulse width modulatedtest signal 502. The pulse width modulated test signal 502 may have arectangular “carrier” frequency. The pulse width modulated test signal502 may be pulse-width modulated with a lower frequency sinusoidalsignal. This example pulse width modulated test signal 502 may drive theswitching element S1 314, as shown in the circuit in FIG. 3, thusapplying the test signal 502 to the battery under test.

To illustrate, where Ft=20 Hz and Np=256, Tperiod=1/(Ft*Np)=195 usec, or5128 Hz. A 5128 Hz sinusoidally pulse width modulated rectangularsignal, if integrated, may yield a 20 Hz sinusoid. In this illustration,the battery under test may be excited by a rectangular signal with apeak value equal to Itest. Because the instantaneous duty-cycle of thisrectangular pulse-train may be modulated by a 20 Hz sinusoid, theaverage current of the test signal, if integrated, may vary at a 20 Hzrate. The RC time constant of the battery itself may perform the firstpart of this signal integration, and the circuitry and software withinthe sensor may provide additional integration.

In an embodiment, the microcontroller can vary the frequency of the testsignal 502 by implementing a phase-increment algorithm. In this process,a frequency variable is added to the lookup table location pointer suchthat every second, third, or nth table location is accessed. Thefrequency variable defines the number of lookup table positions betweeneach sample being output to the PWM. As the frequency variable isincreased, the frequency of the generated signal 502 may increase.

In an embodiment, the current of the integrated sinusoidal frequencytest signal may be adjusted by consistently scaling the values in thelookup table before they are passed to the PWM. For example, if eachtable value is halved before being output to the PWM, then the averagecurrent in the integrated sinusoidal frequency test signal will also behalved, even though the higher frequency peak rectangular current isfixed. Finally, the values in the microcontroller's PWM lookup table canbe altered, if desired, in order to generate any desired waveformincluding sinusoid, triangle, sawtooth, rectangular, or virtually anyother test signal which may be advantageous.

FIG. 6 depicts an example excitation signal 602, as measured from abattery under test. The sensor may measure the excitation signal 602,which results from the pulse width modulated test signal having beenimpressed on the battery. The excitation signal 602 illustrates the ACvoltage that develops across the battery terminals as a result of thepulse-width modulated rectangular excitation current shown in FIG. 5. Atthis point, the excitation signal 602 has been partially filtered due tothe RC characteristics of the battery. The battery, owing to itsbehavior as an RC equivalent circuit, provides a preliminary low-passfiltering function which attenuates some of the high frequency switchingcarrier and begins to reveal the desired low frequency sinusoidalfrequency component.

FIG. 7 depicts an example filtered excitation signal 702. The filteredexcitation signal 702 illustrates the result of fully filtering thevoltage that developed across battery. The filtered excitation signal702 may be measured by the microcontroller.

Further filtering inside the sensor may eliminate or significantlyreduce the residual high frequency switching component, leaving the lowfrequency sinusoidal modulation component as the primary signal to bemeasured. For example, the sensor may include a high-gain amplifier thatis AC coupled (e.g., capacitive coupled) to the battery under test. Theoutput of the high gain amplifier may be fed into the analog-to-digitalconverter in the microcontroller for measurement.

The gain of the amplifier may be controlled by the microcontroller sothat a wide range of battery types with differing internal resistancescan be measured. For example, the amplifier gain may be in the range ofabout 200 to about 1000. Also for example, the total internal resistanceRi=(Ri+Rm) of the battery under test may be equal to 0.005 ohm. Theintegrated component of the low frequency test signal may be equal to 1ampere peak-to-peak. Then, the AC component of Vt may beVt=Itest*Rb=1.0A*0.005 ohm=5 mV peak to peak. This signal may beamplified by a factor of 200, and the result may be a 1.0 voltpeak-to-peak signal that is fed to the analog-to-digital converter inthe microcontroller for measurement.

FIG. 8 depicts an example timing diagram for sampling. Because themicrocontroller may generate the pulse width modulated signalpoint-by-point, the resultant filtered excitation signal also may bemeasured point-by-point. The sensor may sample the waveform 802 atdefined sample points 804. The sample points 804 may correspond to thesignal peaks and valleys. (i.e., the alternating maximums and minimumsof the filtered response signal from the battery under test.)

The microcontroller can determine when to sample the recovered signal atthe signal peaks and valleys. Each signal peak may be a local maximum.Each signal valley may be a local minimum. Via a correlating synchronoussampling detection process the microcontroller can measure the resultantwaveform. Such a process may have a bandpass amplitude response that candiscriminate against unwanted signals at frequencies that differ fromthe sampling rate. The bandwidth of this bandpass response decreases asthe number of averaged samples is increased.

For example, as the microcontroller generates each point in thedigitally synthesized excitation current sinusoid, it measures at thepeaks of the resultant voltage sinusoid. The microcontroller may makemultiple measurements at the sinusoidal signal peaks repetitively andmay perform an arithmetic average of the measurements.

Since the microcontroller is generating the pulse width modulated testsignal while simultaneously measuring the batteries response, themicrocontroller may determine when the peaks of the amplified signalwill occur and measure the precise peak-to-peak value of the amplifiedVt signal. The microcontroller may measure the response signal inaccordance with a signal value that produces a peak. The microcontrollermay be programmed to measure a plurality (e.g., 256) positive andnegative signal peaks. The microcontroller may perform a mathematicalaverage of the measured values.

Although FIGS. 4A-8 show waveforms with differing scale factors, itshould be appreciated that scale factors may be set appropriate for thetype of battery being measured. For example, the rectangular excitationwaveform, as shown in FIG. 5, may switch between zero amps and a peakexcitation current appropriate for the type of battery being measured.In typical applications, this peak current may range from about 1-2amps, and the frequency of the modulating sinusoid may range from 10-30Hz. The peak-to-peak value of the unfiltered (e.g., as shown in FIG. 6)and fully filtered waveform (e.g., as shown in FIG. 7) may be a functionof the peak excitation current and the internal impedance (Z) of thebattery. The value of this voltage will typically follow Ohm's Law:Vbat=Ipeak×Zbat. The admittance of the battery may be calculated as theinverse of the impedance (i.e., Zbat).

FIG. 9 depicts an example flow chart for testing an internalcharacteristic of a battery. At 902, the sensor may generate acharacterization of a frequency spectrum at the battery. The sensor maydetermine a frequency at which to test the battery. Generally, inbattery power-plant systems, the battery charging sub-system, the loadsubsystem, and various other influences often produce noise and otherdisturbances that generate interference. The sensor may characterize thesignal spectrum around the frequency where a battery test is proposed tobe made in order to determine if background noise and interferencelevels are sufficiently low to make an accurate measurement. The sensormay make frequency agile, frequency selective measurements of ACsignals.

The disclosed correlating detection process may be used by the sensor inorder to scan a range of frequencies around a proposed measurementfrequency. For example, the sensor may operate in a receive-only mode,altering the detection sampling frequency in a stepwise manner, making asignal measurement at each sampling frequency. If the frequencyselective detection process is run without generating a correspondingtest signal, the detection process may measure the level of signals atthe sampling frequency. If this receive-only measurement process isrepeated for multiple frequencies within some spectral range, then aspectrum-analysis data set is available. This spectrum analysis data setcan be analyzed to determine a frequency or for measurements thatreduces the effect of interfering signals.

After a range of frequencies are scanned and characterized in thismanner, the sensor may choose a frequency at which the measuredbackground noise is at a minimum and/or sufficiently low to make themeasurement. The sensor may then proceed to operate the FET currentswitch to generate the test signal and make the battery testmeasurements. In an embodiment, the sensor may report the receive-onlyfrequency point measurements to the control unit so that the result canbe displayed as a spectrum analysis display. The sensor may report thebattery test results and battery test data to the control unit.

At 904, sensor may generate a pulse width modulated test signal. Theduty cycle of the pulse width modulated test signal may vary inaccordance with a plurality of alternating current signal data values.For example, the plurality of alternating current signal data values maybe stored in memory as a stepwise approximation of a sinusoidalwaveform. The frequency of the sinusoidal waveform may be selected basedon the results at 902.

At 906, the battery may be excited with the pulse width modulated testsignal. For example, the sensor may apply the test signal to theterminals of a battery under test. The sensor may apply the signal tothe battery under test concurrently with generating the test signal. Forexample, the sensors may drive a controllable switch with the testsignal. The switch when closed may drive a test current across thebattery under test.

The battery may alter the test signal, producing a response signal. Theresponse signal may be a version of the test signal altered due to theRC characteristics of the battery. The battery, owing to its behavior asan RC equivalent circuit, may provide a preliminary low-pass filteringfunction that attenuates some of the high frequencies of the pulse widthmodulated test signal. The resultant response signal may reveal adesired low frequency sinusoidal frequency component. The responsesignal may be further filtered by the sensor.

At 908, the response signal of the battery as excited by the pulse widthmodulated test signal may be measured. The response signal may besampled in accordance with the generating of the test signal. Forexample, the response signal may be sampled via correlating synchronoussampling detection process. The response signal may be measured bysampling at each peak and valley of the response signal by themicrocontroller based on the data that the microcontroller used togenerate the test signal.

Correlated detection measurement may, in effect, produce a frequencyselective, bandpass filtered, detection function on the amplifiedsignal. This detection function may result in the detection of thedesired signal's amplitude while rejecting the high-frequenciesgenerated by the sensor's PWM and other noise sources in the batteryplant system. As more measurements are averaged the detection processmay become more selective and the measurement time may increase.

From the measurements, the microcontroller may determine the RCcharacteristics of the battery. For example, the microcontroller maydetermine the admittance of the battery. The microcontroller may measurethe resultant amplitude to determine the RC characteristics of thebattery including the admittance. The admittance may be used as anindication of the health of the battery.

At 910, it may be determined that the battery is overcharged. If it isdetermined that the battery is overcharged, current may be drawn fromthe battery to correct the overcharged state. For example, themicrocontroller may be used to draw current from the battery. In anembodiment, current may be drawn from a battery selected from a stringof batteries.

The sensor may equalize the charge of an individual battery within amonitored string of batteries. The sensor may measure the DC terminalvoltage of the battery. The sensor and/or sensors may perform a variablecharge equalization of the individual batteries in the string, byimpressing a variable current load on the battery, which may removecharge from the battery.

The central site control unit may monitor the terminal voltages of eachbattery in each string. The site controller can determine whichbatteries might be overcharged and which batteries might beundercharged. The site controller may send commands to the sensors onthe overcharged batteries instructing them to generate a continuousprogrammed test current on the battery until the excess charge isremoved. In this manner, battery life may be extended becauseovercharging, a leading cause of battery failure, may be mitigated.

1. A device for testing an internal characteristic of a battery, the system comprising: a memory to store a plurality of alternating current signal data values; a processor to generate a pulse width modulated signal at the battery and to measure a response signal at the battery, wherein a duty cycle of the pulse width modulated signal varies in accordance with the plurality of alternating current signal data values.
 2. The device of claim 1, wherein the processor measures a peak of the response signal.
 3. The device of claim 1, wherein the processor measures the response signal in accordance with a peak of the plurality of alternating current signal values.
 4. The device of claim 1, wherein the processor averages a plurality of measurements of the response signal.
 5. The device of claim 1, wherein the alternating current signal data values are sinusoid signal data values.
 6. The device of claim 1, wherein the processor calculates the internal characteristic of the battery according to an average of a plurality of measurements of the response signal.
 7. The device of claim 6, wherein the internal characteristic is internal admittance.
 8. The device of claim 1, further comprising a communications interface, wherein the processor transmits a measurement via the communications interface.
 9. The device of claim 8, wherein the processor receives a command to test the battery via the communications interface.
 10. The device of claim 1, wherein the processor measures a voltage of the battery at a plurality of frequencies.
 11. The device of claim 1, wherein the processor generates a characterization of a frequency spectrum and determines a frequency at which to test the battery in accordance with the characterization.
 12. The device of claim 1, wherein the processor determines that the battery is overcharged and draws current from the battery.
 13. A method for testing an internal characteristic of a battery, the method comprising: generating a pulse width modulated test signal, wherein a duty cycle of the pulse width modulated signal varies in accordance with a plurality of alternating current signal data values; exciting the battery with the pulse width modulated test signal; and measuring a response signal of the battery as excited by the pulse width modulated test signal.
 14. The method of claim 13, wherein measuring the response signal comprises measuring a peak of the response signal.
 15. The method of claim 13, wherein measuring the response signal comprises measuring the response signal in accordance with a peak of the plurality of alternating current signal values.
 16. The method of claim 13, further comprising averaging a plurality of measurements of the response signal.
 17. The method of claim 13, wherein the alternating current signal data values are sinusoid signal data values.
 18. The method of claim 13, further comprising calculating an internal characteristic of the battery according to an average of a plurality of measurements of the response signal.
 19. The device of claim 18, wherein the internal characteristic is internal admittance.
 20. The method of claim 13, further comprising transmitting a measurement via a communications interface.
 21. The method of claim 20, further comprising receiving a command to test the battery via the communications interface.
 22. The method of claim 13, measuring a voltage of the battery at a plurality of frequencies.
 23. The method of claim 13, further comprising generating a characterization of a frequency spectrum and determining a frequency at which to test the battery in accordance with the characterization.
 24. The method of claim 13, further comprising determining that the battery is overcharged and drawing current from the battery.
 25. A system for testing a plurality of batteries, the system comprising: a plurality of connected battery sensors, wherein each battery sensor comprises a respective memory to store a plurality of alternating current signal data value and a respective processor to generate a pulse width modulated signal at a respective battery and to measure a response signal at the battery, wherein a duty cycle of the pulse width modulated signal varies in accordance with the plurality of alternating current signal data values; and a control unit in communication with each of the plurality of connected battery sensors, wherein the control unit receives measurement data from each of the plurality of connected battery sensors. 